Known semiconductor devices having TFTs on an insulating substrate made of glass or the like include active-matrix liquid-crystal displays and image sensors which use such TFTs to activate pixels.
Generally, TFTs used in these devices are made of a silicon semiconductor in the form of a thin film. Silicon semi-conductors in the form of a thin film are roughly classified into amorphous silicon semiconductors (a-Si) and crystalline silicon semiconductors. Amorphous silicon semiconductors are fabricated at low temperatures. In addition, they are relatively easy to manufacture by chemical vapor deposition. Furthermore, they can be easily mass-produced. Therefore, amorphous silicon semiconductors have enjoyed the widest acceptance. However, their physical properties such as conductivity are inferior to those of crystalline silicon semiconductors. In order to obtain higher-speed characteristics from amorphous silicon semiconductors, a method of fabricating TFTs consisting of a crystalline silicon semiconductor must be established and has been keenly sought for. It is known that crystalline silicon semiconductors include polysilicon, silicon crystallites, amorphous silicon containing crystalline components, and semi-amorphous silicon that is midway in nature between crystalline state and amorphous state.
Known methods of obtaining these crystalline thin-film silicon semiconductors include:
(1) During fabrication, a crystalline film is directly created.
(2) An amorphous semiconductor film is once formed. Then, the film is irradiated with laser light so that the energy of the laser light imparts crystallinity to the film.
(3) An amorphous semiconductor film is once formed. Thermal energy is applied to the film to crystallize it.
Where the method (1) above is utilized, it is technically difficult to form a semiconductor film having good physical properties over the whole surface uniformly. Also, the film is formed at a high temperature of over 600.degree. C. and so cheap glass substrates cannot be used. Hence, this method presents problems regarding costs.
In the method (2), an excimer laser is used most commonly today. If this excimer laser is employed, the laser light illuminates only a small area and hence the throughput is low. Furthermore, the stability of the laser is not stable enough to uniformly process the whole surface of a large-area substrate. Therefore, we feel that this method is a technique of the next generation.
The method (3) above can process substrates of larger areas compared with the methods (1) and (2). However, a high temperature exceeding 600.degree. C. is also necessary. It is necessary to lower the heating temperature where cheap glass substrates are used. Especially, liquid crystal displays having larger areas have tended to be manufactured today. With this trend, larger glass substrates have to be employed. Where larger glass substrates are used in this way, shrinkage and stress produced during a heating step that is essential for semiconductor fabrication deteriorate the accuracies of mask alignment and other steps. This presents serious problems. Especially, in the case of Corning 7059 which is most commonly used today, the strain point is 593.degree. C. Therefore, if the prior art heating-and-crystallization step is effected, a large distortion is induced. Besides the problem of temperature, the heating time, i.e., the time required for crystallization, poses problems. In particular, the heating time necessary for crystallization is as long as tens of hours or longer in the present process. Therefore, it is necessary to shorten the heating time.